The invention relates to a paper sheet impregnated with a composition in aqueous medium, used as a decorative covering base in the manufacture of decorative panels or profiles. The invention also relates to the impregnation composition and to decorative laminates or profiles containing this sheet.
For many years, decorative panels have been used as materials in dwellings and commercial and industrial buildings. Typical applications of such panels are for covering furniture, table tops, wall surfaces and the like.
Many kinds of decorative panel exist: so-called high-pressure panels, so-called low-pressure panels and panels covered with an adhesive sheet.
The so-called high-pressure panels are produced from a core consisting of sheets, generally made of kraft paper, impregnated with thermosetting resin and more particularly with a phenolic resin.
A decorative sheet is placed over the stack of sheets constituting the core, this decorative sheet generally being a sheet of paper carrying a printed pattern or being colored or containing decorative particles, impregnated with a thermosetting resin which does not turn yellow under heat. In general, a protective covering sheet, known as an "overlay", is placed over the decorative sheet, this overlay having no pattern and being transparent in the final laminate. The stack of impregnated sheets is then placed in a press fitted with a plate which imparts the surface state. The stack is then densified by heating and pressing, so as to obtain a unified structure.
This structure is then attached to a base support; for example it is adhesively bonded to a panel of particles.
So-called low-pressure panels are produced using only a decorative sheet impregnated with thermosetting resin (and optionally an "overlay" sheet) which is laminated directly onto the base support.
The third kind of decorative panel consists of panels composed of a base support, generally a panel of wood particles or agglomerated fibers, and of a sheet of decorative paper impregnated with a composition, attached to the support by means of an adhesive.
The sheet of paper is a decorative sheet of uniform color or one containing decorative patterns. The patterns are generally applied by printing onto the sheet, before or after impregnation. A varnish or a lacquer is also applied, the purpose of which is to protect the surface of the sheet.
These sheets are usually impregnated using a size press or another device for impregnation on a paper machine or away from a paper machine with compositions containing aqueous dispersions based on styrene/ethyl acrylate/butyl acrylate copolymer and an aqueous solution of resins containing formaldehyde, such as melamine/formaldehyde or urea/formaldehyde resins.
The sheets impregnated on-line on the paper machine are said to be preimpregnated and those impregnated off-line are said to be post-impregnated. The invention relates to these two types of impregnated sheet.
A major drawback of these sheets thus impregnated is that they contain formaldehyde which is harmful to the health; the content of this compound is subject to increasingly restrictive legislation.
Another considerable drawback is that their flexibility or ability to be deformed is relatively insufficient, they are fragile to handle and breaks or cracking of the sheet are thus observed when it is applied to a surface which is not flat, that is to say a surface with relief or hollows or one which has ridges.
Moreover, the sheets must have good resistance to peeling, and thus good internal cohesion, in order for them not to become detached from the support on which they are adhesively bonded and also in order for there to be no problems of tearing off or deterioration of these sheets during machining of the panels.
Thus, the sheets must simultaneously be sufficiently flexible and deformable to allow their use as a covering for surfaces which are not flat and to have a high internal cohesion; however, a person skilled in the art knows that it is difficult to obtain these two characteristics simultaneously for a sheet.
The problem to solve is thus one of providing impregnated sheets containing no formaldehyde while at the same time having the characteristics required for its application, in particular a high internal cohesion combined with good deformability.
The sheets must also have good printability and have a good aptitude for lacquering or varnishing.
The French application registered under the number FR 92/07558 describes a composition for impregnating a sheet for decorative covering, which is free of formaldehyde and which contains a styrene/ethyl acrylate/butyl acrylate copolymer and a binder.